The OFFSETOF() macro in C++

In this article, you will learn about the offsetof() macro function in C++ with its syntax and examples.

The <cstddef> or <stddef.h> header contains the offsetof() macro in C++, which is used to find the offset of a given member inside a structure or class. It is helpful when working directly with memory layouts, like data structures or hardware registers. It is an essential component of low-level programming.

Syntax:

It has the following syntax:

'type': The class or structure type that includes the given member.

'member': The member's name for whom the offset needs to be determined.

The member's given byte offset inside the structure or class is returned by the offsetof() macro. Concerning the start of the structure or class, it enables you to determine a specific member's memory address.

1. Purpose

The main goal of offsetof() is to simplify low-level memory management by enabling programmers to determine a member's exact position within a class or structure. It is especially helpful when customized serialization and deserialization or working with hardware registers require manual memory layout management.

2. Common Use Cases

Data Serialization and Deserialization: When working with binary data, programmers can use offsetof() to find where data fields are located inside a structure in preparation for serialization and deserialization.

Hardware Interaction: Use offsetof() function in systems programming or when working with hardware registers to access particular bits or fields within a data structure.

Memory Inspection/Debugging: Developers can use offsetof() function for debugging to examine a structure's memory layout at runtime.

3. Portability

The use of offsetof() might not be completely portable across different compilers or platforms because it relies on the compiler's implementation of structure layout. However, it is widely supported and commonly used in practice.

4. Standard Library Header

The <cstddef> header in C++ or <stddef.h> in C usually contains the offsetof() macro. For this macro to function, the relevant header must be included.

5. Member Accessibility

There is no accessibility check on the supplied member carried out by offsetof() function. With the type and member name supplied, it computes the offset automatically.

6. Usage

i) Memory Layout

Classes and structures in C++ are not guaranteed to have a certain memory layout, and the compiler may add padding between members to ensure alignment.

By considering any padding added by the compiler, developers can ascertain a member's true byte offset using the offsetof() function.

ii) Alignment of Data Structure

Exact control over data structure layout is essential in low-level programming, such as embedded systems and systems programming.

By aiding developers in understanding and managing the memory architecture, offsetof() makes it easier to create tightly packed structures.

iii) Pointer Arithmetic

With a pointer to the structure, offsetof() can be used to determine the memory position of a certain member when working with raw memory or building data structures such as linked lists, trees, or other custom structures.

Example:

Let us take an example to illustrate the use of offsetof() function in C++:

Output:

Offset of Int:   0 bytes
Offset of Char:  4 bytes
Offset of Float: 8 bytes
Offset of Double: 16 bytes
Offset of Short: 24 bytes

Explanation:

1. Header Inclusions

• #include <iostream>: It adds the header for the standard input/output stream to print output using std::cout.
• #include <cstddef>: It includes the header that provides the offsetof()

2. Struct Definition

• struct My_Struct {};: It describes a structure with five distinct data types (int, char, float, double, and short) under My_Struct.

3. Main Function

• size_t offsetInt = offsetof(My_Struct, my_Int);: It uses the offsetof macro to determine the byte offset of the member my_Int inside the My_Struct structure. The offsetInt variable contains the result.
• For other members (my_Char, my_Float, my_Double, and my_Short), offsets are calculated in similar lines.
• std::cout << "Offset of Int: " << offsetInt << " bytes\n";: It prints the calculated offset for the my_Int member.
• Similar lines print offsets for other members.

4. Output

• The program computed and printed the byte offsets of every member of the My_Struct The offsets denote the distance between the starting point of the structure and the corresponding members stored in memory.

Regarding low-level programming chores, the offsetof() macro is useful for developers. It offers a method for exact control over memory organization, which is necessary when memory efficiency, alignment, and direct memory manipulation are crucial, even though its use necessitates careful consideration of portability and structural arrangement.